The stream system of a watershed is divided into reaches, and the watershed into hydrologic units (HU), for the convenience of work as a part of a hydrologic or hydraulic study.

This course provides some details on the selection of reaches for hydrologic or economic studies, presents alternative means for studies of alluvial fans, and briefly describes a hydrologic unit and its use in a study. A reach is a length of stream or valley used as a unit of study. It contains a specified feature that is either fairly uniform throughout, such as hydraulic/geomor-phic characteristics or flood damages, or that re-quires special attention in the study, such as a bridge. Reaches are generally shorter for hydraulic studies than for other types of studies, so it is best to consider hydraulic needs first when selecting reaches and then combine the hydraulic reaches into longer ones for the other studies. 

Topics: Introduction to Stream Reaches Reach Features Estimating Hydraulic roughness (Manning n-value) Reach data input for computer programs National Geospatial Data for stream reaches Project Benefits Estimation on Alluvial Fans